It would have been respectful for the Singer to let you know that he was doing some stuff with the keyboardist. I might even go so far as to say it was the "right" thing, but it isn't strictly necessary.
Tell them you saw the gig and thought they were amazing. Tell them you really want to be up there with them and you are working hard on your chops to get better. And do exactly that.
This isn't a bad idea.
6 bills split 4 ways vs split 2 ways.
This!
It's possible that the keyboardist might have found the rest of the group wanting and just took the singer. Hoewever, my suspicion is that there are at least a few other contributing factors.
1-Keyboardist may not have wanted to commit to a "band".
2-Keyboardist may have just wanted a ready-gig with little/no practice, hence grab a singer and play some tunes he already knows.
3-Money. If he feels he's good enough to carry the day there's no reason to split 4 ways when you can split 2 ways, and there's no reason for your singer to turn down the gig. Though it would have been nice to let you know.
It's not fun for you guys, but for a good keyboardist (especially if he's trying to make some $), it's kind of a no-brainer to just grab a singer and do some tunes rather than jumping whole hog into a band.
A somewhat similar story, after spending a decade with a 9 piece band, my trumpet-playing-sometimes-singing uncle bowed out. He's now putting together a quartet with the intent of only playing corporate gigs and well-paying parties (weddings, etc). In a 4 piece there's much more $ for each member, you have shorter-and-fewer practices and it's far easier to coordinate schedules.
As it turns out he still fills in for the original group now and then, but there's almost never a rehearsal required, he gets hired-gun pay rate, and there's even the good vibes you get from the rest of the group by being the old friend who helps the band out in a pinch.